Matrix printer drive element

ABSTRACT

An improved electromagnet design for use as a dot-matrix printer drive element. A solid winding core is provided with integral pole pieces at both ends. A leaf spring is secured to a first pole piece and extends generally parallel to the axis of the core. A magnetic field shunt piece is secured at the free end of the spring on the side facing the core. A printing wire shoe is also secured to the leaf spring at its free end, and extends away from it. The printing wire is affixed in the shoe and extends back across the axis of the core. In the un-energized condition, there is a gap between the shunt and the second pole piece that is roughly one-half of the desired stroke of the printing wire. The electromagnet is of very low mass, but operates at high speed with a long printing stroke having maximum force at the moment of impact of the printing wire on the printing medium. This design allows a plurality of such drive elements to be radially mounted in a circle with all of the respective printing wires close to the center of that circle. The center of the circle will also coincide with the centerline axis of the printed characters.

United States Patent [191 Matschke et al.

[ 51 Sept. 17, 1974 MATRIX PRINTER DRIVE ELEMENT [75] Inventors: Arthur L. Matschke, Westport,

Conn.; Jack K. Horowitz, Laurelton, NY.

[73] Assignee: TeIe Speed Communications, Inc.,

Long Island City, NY.

[22] Filed: Oct. 25, 1973 [21] Appl. No.: 409,699

[52] US. Cl. 335/274, 197/1 R [51] Int. Cl. H01f 7/13 [58] Field of Search 335/270, 274; 197/1 R [56] References Cited UNITED STATES PATENTS 3,266,418 8/1966 Russo 197/1 R 3,672,482 6/1972 Brumbaugh et al 197/1 R FOREIGN PATENTS OR APPLICATIONS 903,100 8/1962 Great Britain 197/1 R Primary ExaminerG. Harris [5 7] ABSTRACT An improved electromagnet design for use as a dotmatrix printer drive element. A solid winding core is provided with integral pole pieces at bothends. A leaf spring is secured to a first pole piece and extends generally parallel to the axis of the core. A magnetic field shunt piece issecured at the free end of the spring on the side facing the core. A printing wire shoe is also secured to the leaf spring at its free end, and extends away from it. The printing wire is affixed in the shoe and extends back across the axis of the core. In the un-energized condition, there is a gap between the shunt and the second pole piece that is roughly onehalf of the desired stroke of the printing wire. The electromagnet is of very low mass, but operates at high speed with a long printing stroke having maximum force at the moment of impact of the printing wire on the printing medium. This design allows a plurality of such drive elements to be radially mounted in a circle with all of the respective printing wires close to the center of that circle. The center of the circle will also coincide with the centerline axis of the printed characters.

11 Claims, 2 Drawing Figures PAIENIEDSEP I new aazarmao MATRIX PRINTER DRIVE ELEMENT BACKGROUND OF THE INVENTION The present invention relates generally to electromagnets and, more particularly, it relates to electromagnets utilized as drive elements in so-called dotmatrix printers.

A dot-matrix printer is one which forms an alphanumeric character from a plurality of dots by forcing the ends of selected printing wires from an array of such wires into contact with a printing medium, i.e., an inked ribbon, and the recording medium. In one arrangement, the array comprises seven wires in a vertical row which produces a character in a five-pulse sequence, with horizontal movement of the array or recording medium between each pulse. Alternatively, the array may comprise 35 wires arranged in five rows of seven wires each, which will produce a character with a single pulse. As the character printed is of the usual size of typewritten alphanumeric characters, about 0.070 X 0.1 inch, it is appreciated that the individual wires must be of small diameter.

Dot-matrix printers are known having a stationary printing heads and axially moveable carriages as well as vice versa. In either case, it is necessary to provide an electromechanical device which, upon receipt of an appropriate signal, forces the wires required to form any given character or portion thereof against the printing medium. The device of choice is a solenoid. One solenoid can, of course, only drive a single printing wire, so in a 7 X 5 array 35 solenoids are required. Generally, solenoids of the type heretofore employed in this service have an axially movable armature within the core, the print wire being secured to one end of the armature. Energizing the solenoid forces the wire against the printing medium and, at the same time, the armature compresses or tensions a spring. The spring provides the force necessary to return the armature and wire to their retracted positions when the coil is deenergized. In such solenoids, the maximum force is developed at the beginning of a stroke, and decreases as the armature moves away from the center of the flux field.

While much progress has been made in the miniaturization of solenoids for this service, they are inevitably much larger than the printing wire. Thus, the array of solenoids will drive wires of differing lengths through differing angles to reach the printing medium. These angles develop radial loads and varying moments on the wire-bearing track affecting bearing and wire life and reducing velocity for given power. This is not efficient. All wire lengths can be made the same by mounting the solenoids on a sperical surface with the printing head at the center, but this is a difficult and expensive solution. Flexing of the wires poses another problem which causes nonuniformity of printing. This may be lessened but is not eliminated byv providing tubular guides for the wires up to a point near the printing head.

The array of solenoids in close proximity to each other creates additional problems. One of the most serious is overheating. The number of solenoids energized for any given alphanumeric character in a 5 X 7 array will vary between 4 and 22. For a high speed printer producing 200 characters per second this means that the solenoid bank will be energized between 800 and 4400 times per second. This creates a substantial cooling load.

If the chosen array is a single row of seven print wires and associated solenoids, it will be appreciated that a single solenoid may be pulsed as many as five times in the formation of a single character. This means that heat generation and wear rates will be high. To maintain a reasonable printing rate, it will be desired to have the armature travel the shortest possible distance, and maintain the print wires in a near straight line. In a 35 solenoid array, the work required of an individual solenoid is much less, but the printing head will be a much more massive unit, which creates problems in moving the carriage, both from character to character and in carriage return time. Also, a 35 solenoid array will necessarily have many solenoids and associated print wires far removed from the centerline axis of the printed character. This can be as much as 30, and means that substantial energy must be expended overcoming friction of the wire against a containing tube, and this also increases wear.

Closely-spaced solenoids will also have overlapping magnetic fields, and this can cause cross-talk problems which impairs the quality of the printing.

OBJECTS OF THE INVENTION It is a general object of the present invention to provide an improved electromagnet for use in a dot-matrix printer.

A further object of the present invention is to provide an electromagnet for use in a dot-matrix printer that will reduce the total flexure of the printing wires, thus reducing wear and improving efficiency.

A still further object of the present invention is to provide an electromagnet adapted for radial mounting in a dot-matrix printer, thereby improving cooling while having the print wires close together.

Another object of the present invention is to provide an electromagnet for a dot-matrix printer having a print wire closer to the centerline axis of the printed character than in previous designs.

Yet another object of the present invention is to provide a simple, economical and reliable electromagnet of low mass for use in dot-matrix printers.

Various other objects and advantages will become clear from the following description of an embodiment thereof, and the novel features will be particularly pointed out in connection with the appended claims.

THE DRAWINGS Reference will hereinafter be made to the accompanying drawings, wherein:

FIG. I is a side elevation view of a solenoid in accordance with the invention; and

FiG. 2 is a left end elevation view of the solenoid of FIG. 1.

DESCRIPTION OF EM BODIM ENTS The present invention comprises, in essential part, an electromagnet including a fixed core and winding, and two preferably integral pole pieces. A leaf spring is attached to one pole piece but is spaced from the other in the unflexed condition. A field shunt piece is attached to the spring at its free end and closely confines the magnetic field when the solenoid is energized. A print wire shoe is also secured to the free end of the spring, the print wire extending at a right angle to the shoe (and spring) back across the axis of the core.

The print wire and the free end of the shoe are very close to the centerline axis of the character, and the electromagnet core axis is on a radius of a circle with the centerline axis as the cneter. When the electromagnet is energized, the shunt approaches the pole piece and the print wire approaches the printing medium. The magnetic force acting on the shunt increases as the shunt gets close to the pole piece, and impact of the print wire on the printing medium occurs at the point of maximum force. Pulsing the solenoid of course tensions the leaf spring, and after the pulse this spring returns to the netural position, retracting the print wire.

With reference to FIG. 1, the electromagnet comprises a core 12 with two integral pole sections 14, 16, a spring 18 mounted with and adjacent to one pole section, a field shunt 20 and a shoe 22 attached to spring 18. A printing wire 24 is attached at the free or distal end of shoe 22. Core 12 of course carries a winding 26, shown in part.

Core 12 with pole sections l4, 16 must be made of a suitable magnetic material or magnetizable material, as must the field shunt 20. it is preferred that core 12 and pole sections l4, 16 be formed as an integral unit, but this is not necessary. In the embodiment shown, core 12 has a rounded rectangular cross-section. This is not a necessary feature, but provides sufficient flux with minimum mass.

Pole section 14 is located at the outer end of core 12 and comprises a flat plate portion 28 of dimensions suitable to retain the cross-section of winding 26, and a leg 30 at one side, providing a mounting structure. Leg 30 has a machined surface 32 including a slot 33 within which spring 18 is secured, and has a threaded hole 34 for receiving a screw (not shown) for this purpose.

A portion 35 ofthe structure to which electromagnet 10 is mounted is shown in FIG. 1. Surface 32 on structure 35 will be at an acute angle to a plane normal to the centerline axis of printed characters; the size ofthis angle is a function of the geometry of the overall printing head and is not of concern herein. Surface 33 is also machined at a slight angle to the axis of core 12 and establishes the gap 46 between shunt 20 and surface 44 of inner pole section 16, as described below.

The inner surface of plate portion 28 is provided with a step-shaped recess 36 adjacent surface 32 to receive the free end of field shunt 20. This improves magnetic field continuity and provides a mechanical dampener for the moving spring, shunt, shoe assembly on the return stroke when the coil is de-energized.

Spring 18 is a leaf spring made out ofa suitable material such as spring steel or beryllium-copper. It is secured between slot 33 in surface 32 and a milled surface of mount 35 and extends down past lower pole section 16. The field shunt 20 and shoe 22 are secured to spring 18 at its free end by a pair of screws 38, 40. Shunt 20 is essentially a bar of magnetic material extending from a point opposite inner pole section 16 into recess 36. So as not to impede the motion of spring 18, field shunt 20 is undercut therefrom a small distance.

I As shown in FIG. 1 this is done by providing an outwardly-extending shoulder 42 at the mounting end for accommodating screws 38, 40. This permits free movement of spring 18.

lnner pole section 16 includes a surface 44 forming a strike plate spaced from and opposed to the secured end of field shunt 20. Surface 44 is not quite parallel with the axis of core 12, but is rather angled slightly so as to be parallel with the opposing surface of field shunt 20 when the electromagnet is energized, spring 18 rotates through an are from surface 32, and the two surfaces are brought into contact. The gap 46 between surface 44 and field shunt 20 is roughly one-half of the desired length of stroke of each printing wire. The lower surface 48 of pole section 16 is configured to provide required flux strength and minimum mass.

Shoe 22 is also secured to spring 18 by means of screws 38, 40, but on the opposite side thereof to field shunt 20. Shoe 22 extends beyond electromagnet 10 a distance determined by the wire path'geometry of the assembled head. It is important that shoe 22 be rigid (i.e., not subject to flexure) and of sufficient mass for printing requirements. The print wire 24 is secured in the distal end of shoe 22 and extends back across the axis of core 12 to the printing medium.

Thus, when coil 26 is energized, shunt piece 20 is drawn across gap 46 and toward strike plate 44, tensioning spring 18 and moving print wire 24 the required distance into contact with the printing and recording medium (not shown).

In conventional solenoids, the armature moves away from the center of the flux field as the stroke proceeds. As a result, the greatest force is generated at the beginning of a stroke. With the present invention the force increases as gap 46 is reduced, and is greatest at the moment of impact with the printing medium. Again with a conventional solenoid, the armature must move the desired length of the printing stroke plus the distance necessary to accomodate any flexure. With the present invention, the distance travelled by shunt 20 across gap 46 is only a fraction of the distance travelled by print wire 24, because of the larger radius through which the distal end of shoe 22 travels. Not only does this allow a smaller gap 46 for a given length of printing stroke, but it also provides a higher angular velocity for print wire 24 than for shunt 20. These features are all advantageous in dot-matrix printing.

It will be appreciated that a plurality of electromagnets may be mounted radially (on their respective surfaces 33) on a suitable circular printing head carriage 35 with the respective shoes 22 and print wires 24 located very close to the center of the circle. Between shoe 22 and the printing medium, wires 24 will describe an almost parabolic arc. Also, all print wires 24 will be the same length, unless the solenoids are mounted in more than one plane, in which case two lengths will be involved.

A dot-matrix printing head carriage adapted for use with the electromagnets of the present invention is described and claimed in co-pending US. application Ser. No. 407,236 filed Oct. I7, 1973 and assigned to the same assignee as the instant application.

Various changes in the details, materials, steps and arrangement of parts, which have been herein described and illustrated in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as defined in the appended claims.

What is claimed is:

1. An electromagnet for actuating a drive element comprising:

magnetic means including two opposed pole sections and a core extending therebetween;

a coil surrounding said core between said pole sections; leaf spring means secured to one said pole section and extending to the other said pole section; magnetic field shunt means secured to said spring means and extending between but normally spaced from said pole sections; and drive element means also secured to said spring means at the free end thereof. 2. The electromagnet as claimed in claim 1, wherein said drive element means comprises:

shoe means secured to said spring means and extending therebeyond; and print wire means secured in the distal end of said shoe means and extending back across the axis of said core. 3. The electromagnet as claimed in claim 1, wherein said magnetic means comprises a single, integral part.

4. The electromagnet as claimed in claim 1, wherein said field shunt means and said drive element means are secured on opposite sides of said spring means at the free end thereof.

5. The electromagnet as claimed in claim 1, wherein said field shunt means is spaced from said spring means except where secured thereto.

6. A drive element for a dot-matrix printer comprising:

magnetic means including two opposed pole sections and a core extending therebetween;

a coil surrounding said core between said pole sectrons;

leaf spring means secured to one said pole section and extending to the other said pole section;

magnetic field shunt means secured to said spring means and extending between but normally spaced from said pole sections;

said shunt means and the other said pole section defining a gap therebetween when said coil is not energized;

shoe means secured to said spring means at the free end thereof and extending therebeyond; and

print wire means secured in the distal end of said shoe means and extending back across the axis of said core.

7. The drive element as claimed in claim 6, wherein said magnetic means comprises a single, integral part.

8. The drive element as claimed in claim 6, wherein said field shunt means and said shoe means are secured on opposite sides of said spring means at the free end thereof.

9. The drive element as claimed in claim 6, wherein said field shunt means is spaced from said spring means except where secured thereto.

10. The drive element as claimed in claim 8, wherein said one pole section includes a recess adjacent the secured end of said spring, said recess accomodating the free end of said shunt means.

11. The drive element as claimed in claim 6, and additionally comprising a mounting bracket on said one pole section for attachment of said spring means and mounting said drive element. 

1. An electromagnet for actuating a drive element comprising: magnetic means including two opposed pole sections and a core extending therebetween; a coil surrounding said core between said pole sections; leaf spring means secured to one said pole section and extending to the other said pole section; magnetic field shunt means secured to said spring means and extending between but normally spaced from said pole sections; and drive element means also secured to said spring means at the free end thereof.
 2. The electromagnet as claimed in claim 1, wherein said drive element means comprises: shoe means secured to said spring means and extending therebeyond; and print wire means secured in the distal end of said shoe means and extending back across the axis of said core.
 3. The electromagnet as claimed in claim 1, wherein said magnetic means comprises a single, integral part.
 4. The electromagnet as claimed in claim 1, wherein said field shunt means and said drive element means are secured on opposite sides of said spring means at the free end thereof.
 5. The electromagnet as claimed in claim 1, wherein said field shunt means is spaced from said spring means except where secured thereto.
 6. A drive element for a dot-matrix printer comprising: magnetic means including two opposed pole sections and a core extending therebetween; a coil surrounding said core between said pole sections; leaf spring means secured to one said pole section and extending to the other said pole section; magnetic field shunt means secured to said spring means and extending between but normally spaced from said pole sections; said shunt means and the other said pole section defining a gap therebetween when said coil is not energized; shoe means secured to said spring means at the free end thereof and extending therebeyond; and print wire means secured in the distal end of said shoe means and extending back across the axis of said core.
 7. The drive element as claimed in claim 6, wherein said magnetic means comprises a single, integral part.
 8. The drive element as claimed in claim 6, wherein said field shunt means and said shoe means are secured on opposite sides of said spring means at the free end thereof.
 9. The drive element as claimed in claim 6, wherein said field shunt means is spaced from said spring means except where secured thereto.
 10. The drive element as claimed in claim 8, wherein said one pole section includes a recess adjacent the secured end of said spring, said recess accomodating the free end of said shunt means.
 11. The drive element as claimed in claim 6, and additionally comprising a mounting bracket on said one pole section for attachment of said spring means and mounting said drive element. 